Have you ever wanted to destroy the civilization? To unmake it, leave no trace of humankind, watch the world burn? Were you ever in despair so deep, that you wanted not only to no longer exist, but to take everything with you? Have you ever dreamed of ashes and dust and little else? Would you push the red button to end it all?
Reapers were mortal people once... Mortal people brought by tragedy beyond the edge of despair, beyond the edge of hate, beyond the edge of fear, beyond the edge of the apathy... They were brought to the End Of Times, and there, they heard the insidious whispers of one of the Horsemen Of The Apocalypse asking them to work for the obliteration of everything that is. There and then, at the End Of Times, they listened to the whispers and accepted their message and mission of bringing the end of the world. Each reaper was a human whose spirit was broken by the life, the world, and the other people, each one was crudely reassembled by a Horseman into a servant, a herald of the end of times and the extinction of humanity.
For the reapers, the Horsemen are much more than the revelation depicts them to be. They are the very incarnation of various form of destruction. The War is violence, wrath, and hate. The Famine is hunger, deprivation, denial, and apathy. The Pestilence is disease, despair, toxicity, and malice, while the Death is fear, accidents, and entropy itself. Those four powers await at the end of times to reap the world, possibly to reap the existence itself, or maybe only to end the humanity and make place for a new, better creation. For some reapers, there is hope in destruction, a never spoken but expected promise of redemption or rebirth, but others choose their path out of sheer desire for destruction, or utter bleak despair... Neither consider the others to be right.
Some reapers believe they serve no master, they claim that Horsemen do not exist, yet, and the calling they all feel, is their own, sent back in time from the future when the most powerful, the right reaper ascends to become the Horseman... Of course, they are all convinced they are the one who will ascend to become its own and every other reaper of the same calling's patron...
Mortals who accept the reaper's mantle are forevermore linked to their patrons, dreaming of decaying cities, burning battlefields, and desolated lands. They are given strength to serve the Horsemen as long as they are needed: they age slowly, they are more resistant to injuries—in fact they are capable of surviving any wounds that are not immediately lethal (such as devastating damage of hearth muscle, brain, or brain stem, decapitation, being cut in half or shredded to pieces), they can never die of starvation or thirst (though they are still maddening and painful experiences for them), nor will they suffer any serious effects of mundane diseases (though diseases of supernatural origins can be still a major inconvenience, except for reapers belonging to The Horseman of Pestilence). The reapers also come unscathed from accidents, though anyone and anything accompanying them lacks any protection whatsoever. Essentially, anything less than direct and deliberate violence will not end the life of a reaper, even if it might make that life even more of a hell than it was before. What's more, injuries that fail to kill them will heal quickly, and crippled limbs and organs will regrow, though often scarred, misshape, and cancerous in appearance.
Beyond their supernatural vitality, the reapers have other powers at their disposal as well. They can sense mundane and supernatural manifestations that resonate with their patrons—death, violence, fear, apathy, famine, disease, tragic accidents... They can call dark steeds to travel further and faster than humans could hope—usually taking shapes of black or white horses, though occasionally appearing in other forms fitting the surroundings—black cars, gloomy elevators, midnight trains, and others. The more powerful the reaper is, the faster the steed travels cutting the way through space, time, and discarded regions of the creation. As the servants of the oblivion, the reapers can decay and destroy inanimate matter or harm living being with a mere touch. Interestingly, while they damage more complex devices more easily and throughly, the efficiency of this ability peaks for popular modern technologies, and then sharply drops for the most cutting edge, unique, and esoteric technology, such as Large Hadron Collider. Another aspect of their power is ability to sense, interact with, and command ghosts whose causes of death resonate with the reapers' patrons (and reapers serving The Horseman Of Death can exert their power over any ghosts). They can also draw the seals of their patrons, to channel some of their influence, spreading emotions and enhancing manifestations belonging in the specific horseman's purview. A seal of War will spread violence for example, while the seal of Pestilence will increase number of infections or virulence of a toxic spill nearby. The most powerful of the reapers abilities, however, is to take on mantle of their patron for a short time, transforming into the Horseman's semblance. A transformed reaper is a force of destruction with greatly increased physical capabilities and outstanding resistance to harm. A powerful reaper incarnating the Horseman can tear a modern tank apart, or cut it into pieces with spectral weapons while deflecting concentrated fire of its support. A well coordinated strike from multiple units can still take such monstrosity out, preventing the reapers from going on an all out rampage. Usually...
Many of the reapers' powers are fueled with entropy, not the thermodynamic measure, but more esoteric and spiritual manifestation of that concept—the reapers seem to be able to charge themselves in places of death and destruction with excessive entropy and use it to break down the order despite the forces opposing the decay. At the same time, the reapers seem to project and increase local entropy spontaneously, increasing the coincidence of disruptive events and bringing doom to others. This never directly endangers the reaper, though it often messes with their plans and schemes in the long term—while the reapers can suppress them to a degree it requires certain effort and patience to execute.
Entropy can accumulate in certain objects with great historical and destructive meaning, forming artifacts that can be used by reapers. Occasionally, a reaper can either forge such artifact deliberately, or retrieve one from one of the many fragments of the creation that were cast out, desolated, or subject to localized devastation. They often have anachronistic, weird, seemingly-nonfunctional or counter-intuitive shapes, such as oversized scythes, swords with impossible blades, spiked pieces of armor. They all seem to work fine in the hands of reapers, and may often become especially potent when they take the apocalyptic form of their patron. In fact, some of them only manifest when their wielder transforms into the semblance of the Horseman.
Sometimes multiple reapers form doomcults working together toward the shared schemes intended to bring doom and destruction to the world, though their own auras of doom tend to interfere even more than for solitary reapers. Such doomcults often include more or less informed mortals, sometimes convinced with pseudo-messianic or apocalyptic message, sometimes swayed by short-sighted greed, sometimes by desire for vengeance.
Serving the Horsemen is not easy despite all those powers and tools wielded by reapers, however, for the reapers are only given a very general sense of their mission—to destroy the world as we know it. Everything else is maddeningly vague. There are no clear instructions, no coordination for their efforts, no timetable to follow... All reapers are free to pursue their nihilistic goal in any way they like. They will dream the repeating nightmares of obliteration, or war, of decaying bodies, and crumbling cities, but are those encouragements? Instructions? Hints? Warnings? All they can be sure of is the constant longing for destruction, for the end to everything that accompanies them. Each reaper decides for oneself how to bring the end in the way fitting his or her patron—incite arms races and spark wars, undermine economy and bad business practices that will destroy crops world-wide or release deadly pathogens to start epidemies of diseases, both new and those thought long eradicated... Each reaper also feels a struggle within—between the desire to end the world now, and the need for patient, long-term planning. In fact the patience seems to be a crucial virtue for reapers, if the word virtue could be possibly used for the world-destroying villains in the first place, for the quick path leads invariably to self-destruction. A reaper trying to wreak direct havoc oneself will be put down by mortals or other supernatural beings swiftly and forcefully. And yet, their patience is constantly tested. The short and long term plans of various reapers can collide with each other, the aura of doom that surrounds each reaper can often disrupt their own schemes when the reaper is unable to suppress it, and there are also other forces active in the world as well, knowingly or incidentally working at cross-purpose with the reapers, including but not limited to powerful supernatural entities, gods, monsters, and enlightened mortals.